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VIOTA.M

RISC-V VIOTA.M Instruction Details

Instruction ManualR-type

Count set bits in the source mask before each element and write the prefix count to integer vector vd.

Instruction Syntax

viota.m vd, vs2, vm
Operand Breakdown
Destination rd: register receiving the operation result.
Source rs1: register holding the first operand.
Source rs2: register holding the second operand.
VVector PermutationMask Operations
Related Search Terms
VIOTA.M instructionRISC-V VIOTA.Mviota.m syntaxviota.m RVVprefix countRVV mask

Instruction Behavior

VIOTA.M is a RISC-V V mask prefix-count instruction. For each active element i, it counts set elements in source mask vs2 with index less than i and writes the count to vd. It is commonly used for compaction, scatter indexing, or deriving compact indices from a mask.

Quick Understanding & Search Notes

VIOTA.M outputs how many matching bits appear before each position; it does not include the current bit itself.

vs2 is the source mask; output vd is an integer vector with element width determined by current SEW.
vm=0 uses v0 as the execution mask and vm=1 is unmasked; inactive and tail elements follow the current vma/vta policy.
Often combined with mask comparisons, compression, or indexed memory access.
RISC-V Unprivileged ISA Manual: V Standard Extension for Vector Operations

Common Usage Scenarios

Sparse Indexing

Understand this scenario with real code like «vsetvli t0, a0, e32, m1, ta, ma vmseq.vi v0, v8, 0 viota.m v12, v0 # v12[i]=count of zeros before i».

Compress Helper

Understand this scenario with real code like «vsetvli t0, a0, e32, m1, ta, ma vmseq.vi v0, v8, 0 viota.m v12, v0 # v12[i]=count of zeros before i».

Conditional Gather

Understand this scenario with real code like «vsetvli t0, a0, e32, m1, ta, ma vmseq.vi v0, v8, 0 viota.m v12, v0 # v12[i]=count of zeros before i».

Pre-Use Checklist

Syntax Check
  • Confirm the current instruction format is R-type.
  • Confirm the operand order matches the example.
Semantic Check
  • Ensure the destination register usage is compatible with the calling convention.
  • Confirm this is not the lower-level form of a pseudo-instruction expansion.

Pitfalls / Common Confusions

Input vs2 is mask register (bit vector); output vd is data vector (SEW-bit elements).
Prefix count covers indices 0 to i-1 (exclusive); for an active element 0, the count is 0.
Commonly used to derive compact indices for masked scatter or compaction sequences.

FAQ

How does VIOTA.M handle masking?

With vm=0, v0 selects active elements; with vm=1, all body elements participate. Inactive and tail elements follow the current policies.